The demand for rechargeable batteries having ever greater energy density has resulted in substantial research and development activity related to lithium rechargeable batteries. The use of lithium is associated with high energy density, high battery voltage and long shelf life.
Rechargeable lithium-ion batteries are the preferred rechargeable power source for many consumer electronics applications. These batteries have the greatest energy density (Wh/L) of presently available conventional rechargeable systems (ie. NiCd, NiMH, or lead acid batteries). Additionally, because of the higher operating voltage of lithium ion batteries fewer cells need to be connected in series than for these other rechargeable systems. Consequently lithium ion batteries are increasingly attractive for high power applications such as electric bicycles, portable power tools and hybrid electric vehicles. Lithium ion batteries use two different insertion compounds for the active cathode and anode materials. Lithium ion batteries based on the LiCoO2/graphite system are now commercially available. Many other lithium transition metal oxide compounds are suitable for use as the cathode material, including LiNiO2 and LiMn2O4. Also, a wide range of carbonaceous compounds is suitable for use as the anode material, including coke and non-graphetizing hard carbon. The aforementioned products employ non-aqueous electrolytes comprising LiBF4 or LiPF6 salts and solvent mixtures of ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl methyl carbonate, and the like. Again, numerous options for the choice of salts and/or solvents in such batteries are known to exist in the art.
Lithium ion batteries can be sensitive to certain types of abuse, particularly overcharge abuse wherein the normal operating voltage is exceeded during recharge. During overcharge, excessive lithium is extracted from the cathode with a corresponding excessive insertion or even plating of lithium at the anode. This can make both electrodes less stable thermally. Overcharging also results in heating of the battery since much of the input energy is dissipated rather than stored. The decrease in thermal stability combined with battery heating can lead to thermal runaway and fire on overcharge. Many manufacturers have incorporated safety devices to provide protection against overcharge abuse. For instance, as described in U.S. Pat. No. 4,943,497 and Canadian Patent No. 2,099,657 respectively, the present products of Sony and E-One Moli Energy (Canada) Limited incorporate internal disconnect devices which activate when the internal pressure of the battery exceeds a predetermined value during overcharge abuse.
These pressure activated disconnect devices thus rely on battery constructions wherein the internal pressure is maintained below the predetermined value over a wide range of normal operating conditions yet, during overcharge, the internal pressure reliably exceeds said value.
In a conventional cylindrical lithium ion battery as depicted in FIG. 1, a jelly roll 4 is created by spirally winding a cathode foil 1, an anode foil 2, and two microporous polyolefin sheets 3 that act as separators.
The jelly roll 4 is inserted into a conventional battery can 10. A header 11 and gasket 12 are used to seal the battery 15. The header includes an internal electrical disconnect device similar to that shown in the aforementioned Canadian Patent No. 2,099,657 and additional safety devices if desired. Often, a safety vent is incorporated that ruptures if excessive pressure builds up in the battery. Also, a positive thermal coefficient device (PTC) may be incorporated into the header to limit the short circuit current capability of the battery. The external surface of the header 11 is used as the positive terminal, while the external surface of the can 10 serves as the negative terminal.
Appropriate cathode tab 6 and anode tab 7 connections are made to connect the internal electrodes to the external terminals. Appropriate insulating pieces 8 and 9 may be inserted to prevent the possibility of internal shorting. Prior to crimping the header 11 to the can 10 in order to seal the battery, electrolyte 5 is added to fill the porous spaces in the jelly roll 4.
FIG. 2a shows details of a similar header as depicted in FIG. 1. The assembly comprises the following sequence: a cap 20 with vent holes, two nickel rings 21, a rupture disc 22, a locating insulator 23, a weld plate 24 that snap fits into a polypropylene gasket 12. The rupture disc 22 is laser welded to the centre of the weld plate 24. The cathode tab is in turn laser welded to the bottom of the weld plate 24. Therefore, all the current must flow through the small contact area at the centre of the weld plate 24 making the battery hot during charging and discharging. This is undesirable for high power cells because the localized high current densities can generate heat, which is not easily dissipated from such a confined area. Moreover, it is difficult to decouple disconnect pressure from current carrying capability for conventional header design.